On Holy Saturday of 1918, while Alexandrina, Deolinda and a young
apprentice were busily sewing, three men violently entered their home
and attempted to sexually violate them. To preserve her purity,
Alexandrina jumped from a window, falling four metres to the ground.

Also

Like St Maria Goretti, she was ready to die rather than consent to the
man's lustful advances.

I'm sure there is more women who were saints and had similar story.
My question is how does a woman who is raped or forced to an unwanted impure act lose her purity? Does only a virgin keep her purity intact? I personally don't think they would lose their purity even if they "let" them to rape them.

I agree with what the saints did and see it as a heroic act. I just don't understand why do they say "and this way they preserved their purity" like they would lose it if they let them. This tells me that you can only be pure if you stay a virgin. What is the catholic church understanding on this?

It's interesting to note that the Dominican friar Laurent d'Orléans, O.P., in his Somme le roi, whom French king Philip III commissioned in 1279 (not long after St. Thomas's death in 1274), devised a sevenfold ranking of chastity:

how does a woman who is raped or forced to an unwanted impure act lose her purity?

She can keep her virginity.

Consenting to illicit sexual intercourse (such as fornication) is a sin. The Bl. Alexandrina and St. Maria Goretti stories show that they would rather die than sin. Sin requires free consent of the will; no one can be forced to sin.

If a virgin is violated, she does not forfeit the aureole, provided she retain unfailingly the purpose of observing perpetual virginity, and nowise consent to the act. Nor does she forfeit virginity thereby; and be this said, whether she be violated for the faith, or for any other cause whatever. But if she suffer this for the faith, this will count to her for merit, and will be a kind of martyrdom: wherefore Lucy said: "If thou causest me to be violated against my will, my chastity will receive a double crown"*; not that she has two aureoles of virginity, but that she will receive a double reward, one for observing virginity, the other for the outrage she has suffered. Even supposing that one thus violated should conceive, she would not for that reason forfeit her virginity: nor would she be equal to Christ's mother, in whom there was integrity of the flesh together with integrity of the mind.[*Office of S. Lucy; lect. vi of Dominican Breviary, December 13th]

ok, so they should stop writing: "and this way they preserved their purity" because clearly they would preserve it even if the result was different since there wasn't a consent.
– GrasperMar 4 at 17:43

@Grasper How do you know "clearly they would preserve it even if the result was different"? Jumping out of the window was probably the only way Bl. Alexandrina could preserve her purity; she might have been worried she might consent if she didn't flee, fleeing being the best way to avoid sins against purity. She probably knew she couldn't physically resist him, let alone resist his temptations. She probably wasn't physically strong enough to chase him out like St. Thomas Aquinas did his seductress with a firebrand. One must flee occasions of sin; failure to do so is itself a sin.
– GeremiaMar 4 at 17:56

Are you saying if she didn't to jump out the window she would actually wanted to be raped? What about her sister and the friend? They lost their purity because they didn't jump out the window. She left her sister inside that would bother me more than my own life. I know we don't know the whole story, I don't want to judge her, clearly she is a saint. Anyway: your answer is not answering my question whether one can be pure without being a virgin.
– GrasperMar 4 at 19:23

@Grasper I'm addressing "how does a woman who is raped or forced to an unwanted impure act lose her purity?" by showing she can keep her virginity. "Does only a virgin keep her purity intact?" / "whether one can be pure without being a virgin" seems to be a separate question. Which question are you asking?
– GeremiaMar 4 at 20:52